Noninvasive characterization of a flowing multiphase fluid using ultrasonic interferometry

ABSTRACT

An apparatus for noninvasively monitoring the flow and/or the composition of a flowing liquid using ultrasound is described. The position of the resonance peaks for a fluid excited by a swept-frequency ultrasonic signal have been found to change frequency both in response to a change in composition and in response to a change in the flow velocity thereof. Additionally, the distance between successive resonance peaks does not change as a function of flow, but rather in response to a change in composition. Thus, a measurement of both parameters (resonance position and resonance spacing), once calibrated, permits the simultaneous determination of flow rate and composition using the apparatus and method of the present invention.

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy to The Regents ofThe University of California. The government has certain rights in theinvention.

FIELD OF THE INVENTION

The present invention relates generally to swept frequency acousticinterferometric (SFAI) determination of sound velocity and absorption influids and, more particularly, to the use of SFAI to noninvasivelydetermine flow velocity and composition for flowing fluids.

BACKGROUND OF THE INVENTION

Swept frequency acoustic interferometry (SFAI) [1] is an adaptation ofthe techniques of ultrasonic interferometry developed several decadesago for determining sound velocity and absorption in liquids and gases.In the original technique, and also in more recent modifications of thetechnique [2], the transducers (sensors) were placed in direct contactwith the fluid being tested. This restricted the use of this techniqueto highly specialized laboratory characterization of fluids. Bycontrast, the SFAI technique extends the capabilities of the ultrasonicinterferometry technique significantly and allows the noninvasivedetermination of velocity and attenuation of sound in a fluid (liquid,gas, mixtures, emulsions, etc.,) inside sealed containers (pipes, tanks,chemical reactors, etc.) over a wide frequency range. In addition, ifthe container material properties (density and sound velocity) areknown, the liquid density can be determined using the SFAI technique. Ithas also been shown that it is possible to uniquely identify variouschemical compounds and their most significant precursors based on thephysical parameters of sound: velocity, attenuation, frequencydependence of sound attenuation, and density [3].

Oil companies have recently shown interest in noninvasive techniques forcharacterizing oil flow in pipes from oil fields.

U.S. Pat. No. 5,606,130 [4] states that it is anticipated that the SFAImeasurements described therein can be performed on flowing samples inpipes. However, no mention is made therein of how to perform suchmeasurements.

Accordingly, it is an object of the present invention to provide anapparatus and method for determining the composition of flowing fluids.

Another object of the invention is to provide an apparatus and methodfor determining the flow rate of a fluid.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention, as embodied and broadly describedherein, the method for monitoring the composition of a fluid flowingthrough a vessel hereof includes the steps of: applying a continuousperiodic acoustical signal to the outside of the vessel such that theacoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; detecting the vibrational features generated in theflowing liquid; sweeping the continuous periodic acoustical signalthrough a chosen frequency range which includes two chosen consecutivemaxima among the vibrational resonance features; and measuring thefrequency difference between the two chosen consecutive maxima of theflowing fluid, whereby changes in the composition of the fluid areidentified.

In another aspect of the present invention, in accordance with itsobjects and purposes, the method for monitoring the flow rate of a fluidthrough a vessel hereof includes the steps of: applying a continuousperiodic acoustical signal to the outside of the vessel such that theacoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; detecting the vibrational resonance featuresgenerated in the flowing liquid; sweeping the continuous periodic signalthrough a chosen frequency range which includes two chosen consecutivemaxima in the standing-wave vibrational pattern; recording the frequencydifference between the two chosen consecutive maxima to determinewhether the composition of the fluid has changed; correcting thelocation of the resonance peaks in response thereto; and determining thefrequency of one chosen resonance peak, whereby the flow rate of thefluid is determined.

In yet another aspect of the present invention, in accordance with itsobjects and purposes, the method for monitoring the composition of afluid flowing at a flow rate through a vessel hereof includes the stepsof: applying a continuous periodic acoustical signal to the outside ofthe vessel such that the acoustical signal is transferred to the flowingfluid, thereby generating vibrational resonance features having aplurality of maxima and minima therein; detecting the vibrationalfeatures generated in the flowing liquid; sweeping the continuousperiodic acoustical signal through a chosen frequency range whichincludes one maximum among the vibrational resonance features; measuringthe flow rate of the fluid; measuring the frequency of the maximum ofthe flowing fluid; and correcting the frequency of the maximum for theflow rate of the fluid, whereby changes in the composition of the fluidare identified.

In still another aspect of the present invention, in accordance with itsobjects and purposes, the method for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel hereof includes thesteps of: applying a continuous periodic acoustical signal to theoutside of the vessel such that the acoustical signal is transferred tothe flowing fluid, thereby generating vibrational resonance featureshaving a plurality of maxima and minima therein; detecting thevibrational features generated in the flowing liquid; sweeping thecontinuous periodic acoustical signal through a chosen frequency rangewhich includes one maximum among the vibrational resonance features;measuring the frequency of the maximum of the flowing fluid; determiningthe composition of the fluid; and correcting the frequency of themaximum for the composition of the fluid, whereby the flow rate of thefluid is determined.

Benefits and advantages of the present invention include the noninvasivemeasurement of flow rate and changes in composition of a flowing fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate an embodiment of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 a is a schematic representation of one embodiment of theapparatus of the present invention showing a dual-element transducerlocated on one side of the pipe or tube through which the liquid flows,FIG. 1 b shows a second embodiment of the apparatus of the presentinvention showing the transmitting transducer on one side of the pipe ortube and the receiving transducer on the other side thereof, and FIG. 1c shows a third embodiment of the present invention, wherein a singlepiezoelectric transducer is used for both generating an oscillatorysignal in the sample and for responding to the resonances producedthereby.

FIG. 2 shows an example of an electronic circuit suitable for observingthe resonance response of the fluid flowing through the tube or pipe asa function of frequency; a similar apparatus would be suitable forobserving changes in the phase of the fluid from that of the initialultrasound signal impressed upon the tube or pipe by the transmittingtransducer as a function of changes in fluid composition or flow rate.

FIG. 3 is a composite resonance spectrum for a noninvasive measurementusing a swept frequency apparatus and method of the present invention,and illustrates that liquid peaks can be studied independently of theresonances induced in the wall of the container if an appropriatefrequency region is selected.

FIG. 4 is a graph of the physical properties of several liquids measuredin a static container.

FIG. 5 shows swept frequency acoustic interferometry measurements madeunder flowing conditions, showing that the sound speed which is relatedto the spacing between the peaks for consecutive resonance does notchange as a result of the flow, nor does the sound attenuation which isrelated to the width of the resonance peaks.

FIG. 6 shows swept frequency acoustic interferometry measurements madein a liquid which contains bubbles; again, the spacing between the peaksdoes not change.

FIG. 7 is a graph of the measured differential phase magnitude as afunction of mass flow for water.

FIG. 8 shows the resonance patterns for water and oil as a function offrequency and illustrates that at an appropriate frequency the resonancepeak characteristics are sensitive to the acoustic properties of theliquid.

DETAILED DESCRIPTION

Briefly, the present invention includes apparatus and method fornoninvasively monitoring both the flow and/or the composition of aflowing fluid using ultrasound. In what follows, fluid will be definedas a liquid, including liquids with more than one constituent, liquidswith some particulates and those containing gas bubbles. As will bedescribed in detail hereinbelow, it was found that the position of theresonance peaks for a fluid excited by a swept-frequency ultrasonicsignal change frequency both in response to a change in composition andin response to a change in the flow velocity thereof. Additionally, thefrequency difference between successive resonance peaks does not changeas a function of flow, but rather in response to a change incomposition. Thus, a measurement of both parameters (resonance positionand resonance spacing), once calibrated, permits the simultaneousdetermination of flow rate and composition using the apparatus andmethod of the present invention. Additional parameters useful fordetermining the fluid composition include the full-width-at-half-maximumof a resonance feature, the amplitude ratio and the acoustic impedanceof the liquid. None of these parameters was found to changesignificantly as a function of flow rate. The apparatus was tested usingdecane, dodecane, water, and brine solutions to determine whether thesecompositions readily distinguishable using the swept frequency acousticinterferometry (SFAI) technique that has been described in detail forstatic fluids in U.S. Pat. No. 5,767,407[1] and U.S. Pat. No. 5,886,262[5], the teachings of both references being hereby incorporated byreference herein.

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Similar or identical structure are labeled usingidentical callouts. Turning now to FIG. 1 a, a schematic representationof one embodiment of the apparatus of the present invention is shownillustrating a dual-element transducer or two, single-elementtransducers 10 a and 10 b, located on one side of the pipe or tube, 12,through which fluid, 14, flows, and electronics, 16, provide the fixedor variable acoustic driving frequency, 18, and receive the resonancesignal, 20, generated in fluid 14. FIG. 1 b shows a second embodiment ofthe apparatus of the present invention showing transmitting transducer10 a powered by swept sine-wave generator, 20, on one side of pipe ortube 12 and receiving transducer 10 b in electrical connection withreceiving and analyzing electronics, 22, on the other side thereof.Examples of the circuitry and principles of operation are found in thedescription for the '232 patent, supra. For single-frequency excitationof resonances within the fluid 14, the change in phase can be monitoredby the apparatus. As will be demonstrated hereinbelow, tube or pipe 12can be fabricated from metals, plastics or glass. FIG. 1 c shows a thirdembodiment of the present invention, wherein a single piezoelectrictransducer, 24, is used for both generating an oscillatory signal in thesample and for responding to the resonances produced thereby. As is alsodescribed in the description for the '262 patent, supra, bridge circuit,26, is employed to derive a differential signal and includes one armwhich contains transducer 24, a balancing arm which contains a matchingor equivalent circuit for the transducer, and a swept sine-wavegenerator. When the transducer is not attached to the pipe, the outputis zero; however, when attached to the pipe, a changing pipe impedancedue to standing waves generated therein generates a signal of one armrelative to that of the other arm and the output is the differencebetween these values.

For measurement of the flow rate, it is necessary to correct for changesin the composition of the fluid, or at least have the knowledge that thecomposition is not changing. There are numerous commercially availablecomposition monitoring devices including real-time, on-line devices suchas infrared spectrometers, and uv/vis spectrometers, as examples, andsampling devices such as liquid chromatographs and mass spectrometers asexamples. One might take a sample for analysis using a syringeintroduced through a septum for off-site analysis. Another procedurewould be to stop the flow and utilize the SFAI procedure detailed inReference 1. Due to the number and variety of these methods, FIGS. 1 a-1c do not show any devices for monitoring the composition of the flowingfluid; except for those taught by the present claimed invention.Similarly, for monitoring the composition of the fluid; certainembodiments of the present invention require that a correction to theresonance peak location for the fluid flow rate be made, or at leastknowledge that the flow rate is constant. There are numerous and variedcommercially available flow measuring devices, some disposed in theinterior of a pipe through which the fluid is flowing, and othersdisposed on the exterior of the pipe. Again, no flow measuring devicesare illustrated in FIGS. 1 a-1 c except for those taught by the presentclaimed invention, for the same reason as the fluid compositionmonitoring apparatus is not displayed.

FIG. 2 shows an example of an electronic circuit suitable for observingthe resonance response of the fluid flowing 14 through the tube or pipe12 as a function of frequency; a similar apparatus would be suitable forobserving changes in the phase of the fluid from that of the initialultrasound signal impressed upon the tube or pipe by the transmittingtransducer as a function of changes in composition of the fluid and/orchanges in fluid flow rate. The electronic circuit comprises a directdigital synthesizer (DDS) IC, 28, for generating frequencies up to 10MHz; amplifier, 30, for amplifying the output signal of transducer 10 b;phase detector, 32, for providing a voltage output proportional to thedifference in phase between two sine-waves; analog-to-digital (A/D)converter, 34, having a minimum of two-channel multiplexing capability,36 MUX (multiplexer); microcontroller, 38 having floating pointcalculation and fast Fourier transform (FFT) capability; and displayunit, 40, for displaying the results. The two transducers utilized werecommercial, off-the-shelf piezoelectric transducers (PanametricVideoscan 5 MHz center frequency, 0.5 in. diameter transducers). Theactual brand is not critical to the measurements and almost anytransducer can be employed.

Microcontroller 38 is software programmable and controls DDS 28 togenerate sine-waves having a chosen frequency within the frequency rangeof the device. The frequency output of DDS 28 can either be fixed orvaried with time (that is, swept). The frequency resolution of theapparatus used to demonstrate the present invention was better than 0.1Hz. The frequency could be swept over a chosen frequency range in afraction of a second.

The output of the DDS is used to excite the transmitter transducer 10 aplaced in physical contact with pipe 10 through which liquid 14 can beflowing. Second transducer 10 b is used as the receiver. It is alsopossible to use a single transducer and measure the impedance changethereof (FIG. 1 c hereof) to make the same type of measurement. However,for the present description, it is simpler to discuss the two separatedtransducer embodiment which are placed in physical contact with pipe 12either on the same side thereof in the vicinity of one another or onopposite sides of pipe 12. Receiver transducer 10 b receives the signalresponse of the fluid/pipe to the excitation signal from transducer 10 awhich is amplified by amplifier 30 with a gain of up to 60 dB. Theamplified signal is processed using multiplexing input 36 of A-Dconverter 34. Microcontroller 38 controls the switching of multiplexer36 input and the data output from A-D converter 34.

For phase measurements, phase detector 32 circuit is employed having asits output the phase difference between the signal to transmittertransducer 10 a and the amplified signal of receiver transducer 10 b.Typically, phase measurements are made at a fixed frequency thatcorresponds to a resonance peak when there is no liquid flow through thepipe. When the liquid is allowed to flow, the phase detector output isrelated to the magnitude of the flow. There is no simple relationship todescribe the phase difference as a function of flow and a calibration isrequired. The observed phase difference is an approximately linearfunction of the flow (see FIG. 7 hereof). Microcontroller 38 cancontinuously monitor the phase output and convert this to a flow valueand display the results using display 40.

For fluid composition monitoring, the circuit switches to the channelthat directs the amplified receiver transducer signal output to A/Dconverter 34. For this measurement, the frequency applied to thetransmitter transducer is rapidly swept through a chosen frequencyrange. This range depends on the dimensions of the pipe (see FIG. 3hereof). Although any convenient frequency range may be employed, it ispreferred that a frequency range between two successive wall resonances(see FIG. 3 hereof) be used. This produces a flat baseline and theresults can be fitted to a theory involving simple equations. A briefdescription of the relationships follows.

As stated hereinabove, in order to readily obtain the acousticalproperties of a fluid, it is convenient to select a measurementfrequency range to avoid resonance contributions from the walls(approximately 4, 6, and 8 MHz in FIG. 3 as examples). To first order,this reduces the analysis essentially to that of sound transmissionthrough a one-layer model making the calculations more straightforwardwithout introducing substantial errors in the measurement of sound speedand sound attenuation. This is similar to avoiding the transducercrystal resonance frequency region in traditional interferometry. Theintensity transmission coefficient, T, for the case of a single fluidlayer having path-length, L, attenuation coefficient, α_(L)(α_(L)L<<11), and sound speed, c_(L), between two identical wallboundaries can be expressed as $\begin{matrix}{{T = \frac{1}{\left( {1 + {\frac{1}{2}\sigma\quad\alpha_{L}L}} \right)^{2} + {\frac{\sigma^{2} - 4}{4}{{Sin}^{2}\left( {\frac{\omega}{c_{L}}L} \right)}}}},} & (1)\end{matrix}$where, σ=Z_(w)/z_(L)+z_(L)/z_(w), ω=2πf, is the angular frequency, andz_(w) and z_(L) are the acoustic impedance of the wall and fluid,respectively. For most liquids inside a metal container, σ≈z_(w)/z_(L).T in Eq. (1) is a periodic function of ωL/c_(L) and reaches a maximum(peak) value whenever the condition 2πf_(n)L/c_(L)=nπ is satisfied,where f_(n) is the frequency of the n-th peak. From this condition, thesound speed c_(L) (c_(L)=2LΔf) can be determined if the frequencydifference between successive peaks is measured.

As stated, the sound speed in the fluid is determined from the frequencyspacing between any two consecutive peaks. Therefore, one needs to sweepthe frequency over a range that encompasses any two successive resonancepeaks. The digitized data of two resonance peaks can then be used toextract the sound speed since the liquid path length (the diameter ofthe pipe) is known. This is the most expedient manner for determiningthe sound speed in the fluid, and the measurement can be made in afraction of a second. If either greater accuracy or resolution isrequired, a second approach may be used. In this approach, a much largerfrequency range is covered such that multiple resonance peaks (say, 10)are observed. The microcontroller is used to perform a FFT of the datawhich determines the periodicity of the resonance peaks which isdirectly related to the peak spacing. This is equivalent to averagingthe sound speed measurement over multiple peak spacings.

Sound attenuation and liquid density are related to the frequencyspectrum. The ratio of transmission coefficient minima, T_(min), andmaxima, T_(max), can be expressed in terms of σ and α_(L) as:$\begin{matrix}{\frac{T_{\min}}{T_{\max}} = {\frac{2}{\sigma} + {L\quad{{\alpha_{L}\left( f^{2} \right)}.}}}} & (2)\end{matrix}$Equation (2) illustrates that both α_(L) and σ can be determined from alinear fit of the data of the transmission ratio factor as a function off². The intercept at zero frequency is related to the acoustic impedanceratio σ. If the impedance of the wall material is known, the liquiddensity can be determined since the sound speed of the fluid isindependently determined as discussed hereinabove.

Another for determining the sound attenuation coefficient is to utilizethe half-power bandwidth of observed resonance peaks. From Eq. (1), aninverse solution for the half-power bandwidth, δf, can be derived interms of acoustic properties of the fluid according to $\begin{matrix}{{\delta\quad f} = {\frac{2c_{L}}{{\pi\sigma}\quad L} + {\frac{c_{L}{\alpha_{L}\left( f^{2} \right)}}{\pi}.}}} & (3)\end{matrix}$Similar to Eq. (2), the second term is the contribution from liquidsound absorption and is identical to the solution obtained fromresonator theory of transducers in direct contact with the liquid. Thefirst term, the width extrapolated to zero frequency δf₀, is independentof frequency and depends on σ, c_(L), and L. This term results from thereflection loss at the wall-liquid interface due to acoustic impedancemismatch and can be used to determine liquid density if the acousticimpedance of the wall is known. This analysis can be used to extract theabsolute value of the sound absorption of the liquid. More often,monitoring the peak width for the resonance peaks for say oil and water(see FIG. 8 hereof) to obtain qualitative discrimination is sufficient.The resonance width is the full-width-at-half-maximum of the peak, andthe microcontroller can rapidly calculate this quantity by fitting thetop part of any peak with a Lorentzian line shape. The Lorentzian can belinearized by inverting (taking the reciprocal of the amplitude at eachfrequency) the data and then a simple parabolic fit is all that isnecessary instead of a nonlinear least-squares fitting. The widthindicated for crude oil in FIG. 8 is meant only for qualitativedescription.

Thus, the spectrum contains all the information related to the liquid,any desired parameter can be extracted through simple calculations.

The above description covers the behavior of sound transmission througha fluid path as a function of frequency and Eq. (1) described thefrequency spectrum. It is possible to least-squares curve-fit thisequation to observed experimental data to extract various parameters ofthe fluid, such as sound speed, sound absorption, and density. However,it is also possible to derive the same information with good accuracyeven by monitoring a single resonance peak. In practice, one oftenrequires to monitor the change in the quality of the fluid, in terms ofsound speed, sound absorption and density variation, flowing through apipe and not absolute values of these quantities. In such a situation,an electronic circuit simply selects and tracks a single resonance peakand measures the peak width, peak position, and the minimum value (thebaseline) of the resonance curve.

If the sound speed of the fluid changes, the selected resonance peakposition will change in frequency. This frequency shift (Δf_(s)) isrelated to the sound speed variation (Δc) simply as Δf_(s)=(n/2L)Δc.Here n, is the order number of the particular resonance peak. Similarly,if the sound absorption of the liquid changes then the observeddifference in full-width at half-maximum (δf) value of the resonancepeak for a selected resonance peak Δ(δf) provides the change in soundabsorption as Δ(δf)≈(Δc/π)Δα. Another method for determining the changein sound absorption is to measure the ratio of the resonance peakminimum, T_(min) to the peak maximum, T_(max) for a single resonancefeature. The change in absorption Δα=Δ(T_(min)/T_(max))/L (see Eq. (2)and FIG. 8 hereof). This approach provides a more rapid procedure fordetermining Δα and does not require curve fitting.

Finally, the variation in the minimum (T_(min)) of the resonance curvecan provide a measure of the variation in the change in acousticimpedance of the liquid. The relationship between the two parameters canbe expressed as ΔT_(min)=(2/Z_(w))ΔZ, where ΔZ is the change in acousticimpedance of the liquid. All these relationships are derived from Eq.(1), and are shown graphically in FIG. 8 hereof for descriptionpurposes. For qualitative monitoring of variation in sound absorption,for example, for simple discrimination between oil and water, it isexpedient to simply determine the width of the resonance peak betweenthe maximum and minimum of the resonance as shown in FIG. 8. Foraccurate determination of sound absorption, it is better to fit theresonance spectrum with several peaks using Eq. (1).

By employing a phase-locked-loop circuit that simultaneously monitorsboth the resonance peak position of a single peak and the peak width inthe most sensitive frequency region, both sound speed and soundattenuation are provided continuously. These values are then used tocharacterize the fluid as in the case of the SFAI. With an additionalcircuit, the density of the liquid can be monitored. The phase-lockingis accomplished by using a saw-tooth wave signal to vary the frequencyof the excitation transducer around the desired resonance frequency of asingle resonance peak. The resonance peak is monitored as a function oftime and provides a measure of the sound speed because the pipe diameteris known. In this case, it is not necessary to determine the frequencyspacing between any two consecutive peaks because the resonance spectrumis determined by the path length (pipe or tube diameter) and the soundspeed of the liquid. Therefore, the position of a single known peakdetermines the sound speed. The output of the frequency modulation is asignal that is amplitude modulated as it is swept through a resonancepeak. If the resonance is sharp then the amplitude modulation over theshort frequency sweep region is of high amplitude with a high medianamplitude value. For low amplitude or wide resonance peaks, the outputsignal is of lower median value with lower amplitude excursions.Therefore, by measuring the RMS value of the signal and AC coupling itso that the DC median value is filtered out, it is possible to derivethe resonance peak width. The median DC value provides a measure of theliquid density.

FIG. 3 is a composite resonance spectrum for a noninvasive measurementusing the swept frequency apparatus and method of the present inventionon a container having a finite wall thickness, and illustrates thatliquid peaks can be studied independently of the resonances induced inthe wall of the container if an appropriate frequency region isselected. The following graph shows what a typical spectrum looks likewhen a swept frequency measurement is made from outside a container witha finite wall thickness.

FIG. 4 is a graph of the physical properties of several liquids measuredin a static container. Decane and dodecane were investigated since bothof these liquids are known to have similar properties to those for oil.Acoustically, these liquids are far apart. The sound speed andattenuation values are summarized in the TABLE. TABLE Sound DensityAttenuation Liquid speed m/s g/cm³ Np m⁻¹ s² × 10¹⁴ Decane 1263 0.73 5.7Dodecane 1300 0.75 6.3 Water 1483 1.00 2.5 Water + 18% (NaCl) 1550 1.016.0 Water + 26% (NaCl) 1585 1.02 30.0

The resolution for sound speed for the SFAI technique of the presentinvention is approximately +2 m/s; this can be improved to 0.1 m/s, ifnecessary. This difference between decane and dodecane permits them tobe identified. Differentiating between water, brine and decane (ordodecane) is straight forward. The same data are presented in a3-dimensional graph in FIG. 3 for clarity.

Recent studies on the sound speed in pure hydrocarbons and mixturesusing the traditional pulse-echo technique by Wang and Nur [5] show thatsound speed in 13 n-alkanes, 10 1-alkenes, and 3 napthene hydrocarbonsamples show that the sound speed decreases linearly with temperaturewith slopes ranging from −3.43 to −4.85 [m/s]/° C. in a temperaturerange between −12° to 132° C. Therefore, if the temperature is known,the sound speed can be corrected for temperature. In a separate study[6] it is shown that the sound speed c for hydrocarbons can be expressedas a function of temperature T and molecular weight M in atomic massunits as:${c = {c_{0} - {\left( {0.306 - \frac{7.6}{M}} \right)T\quad{where}}}},{c_{o}\quad{is}\quad a\quad{{constant}.}}$This shows that it should be possible to identify various hydrocarbonsusing sound speed if this quantity can be measured accurately.

In addition to sound speed, the SFAI technique can also determine soundabsorption in the fluids, which provides an additional physicalparameter for oil characterization. Hydrocarbons also show pronouncedfrequency dependent sound absorption. The SFAI technique of the presentinvention is capable of this type of measurement as well.

A flow loop was employed to perform SFAI measurements under flowingconditions. A 4.5-in. diameter plastic tube was used in the flow loop.Water was used for the liquid because it is easier to work with thancrude oil. The measurement was also performed with vegetable oil. FIG. 5shows the measurements under flowing conditions between 0 and 20gal./min. of water. The spacing between consecutive resonance peaks isseen to be the same for flowing and non-flowing water. This indicatesthat the sound speed does not change when the liquid is flowing. Thewidth of the resonance peaks are also observed to be the same,indicating that sound attenuation also remains invariable under flowingconditions. The difference between the two spectra is a slight shift ofthe entire pattern in frequency.

It is believed by the present inventor that the frequency shift is dueto a slight variation in the acoustical properties of the fluid due tothe flow boundary layer formed adjacent to the inner surface of thewall. This boundary layer tends to introduce a phase shift of the soundwaves reflecting from the wall which can affect the standing-wavepattern formed inside the total fluid path length. The baseline drift tohigher amplitude toward the higher frequency side of the figure is aresult of the fact that the data presented are somewhat close to a wallresonance peak (see FIG. 3 hereof). It has been observed that theconstancy of the sound speed is observed from the FFT of the data.

FIG. 6 shows that SFAI measurements required for determining sound speedcan be made with fluids containing bubbles of gas. For this measurement,nitrogen gas was bubbled through the bottom of a Plexiglas tube about2-in. in diameter, and the measurements were made by attaching twotransducers on the outside of the tube. To be noted is that that thefrequency spacing between consecutive resonance peaks does notsignificantly change, and that the spectra can be clearly observed (themeasurements were made with little (˜1 ms) integration time); moreover,the periodicity can still be determined at relatively high bubblingrates. This indicates that the sound speed does not change appreciablyuntil the volume fraction of bubbles is large when the bubbling rate istoo high. If the integration of the measurement is increased by a factorof 10, the signal-to-noise ratio of the data was found to improveconsiderably, and the observed pattern for the bubbling liquid was foundto be similar to the same liquid without introduced bubbles. This isbecause all the fluctuations due to the bubbles in the measurements areaveraged out, and up to a certain bubble rate, the SFAI measurements arestill quite reliable.

FIG. 7 is a plot of the shift in phase angle as a function of mass flow,demonstrating that the apparatus of the present invention is useful as anoninvasive flow meter; that is, by attaching transducers to the outsideof an existing pipe, the flow of the fluid therein can be monitored.

For real-time (continuous) monitoring, it has been found to be mostuseful to select a single resonance peak at an appropriate frequency.FIG. 8 is a plot of resonance amplitude as a function of frequency forcrude oil (upper trace) and for water (lower trace) in a 2-inch diameterglass pipe. In the frequency range between 3.78 and 3.8 MHz (enclosed bythe rectangle), the particular resonator cavity (the inside of the pipe)reaches its maximum sensitivity in terms of monitoring changes in soundspeed. There are many such frequencies dispersed in a regular manner. Afrequency shift of 5 kHz is observed between the data for crude oil andwater. The SFAI technique of the present invention can easily resolve 1Hz, therefore, allowing a sound speed resolution of 1 part in 5000.Besides the shift in frequency, the resonance width also changesdramatically which indicates a large variation in sound absorption. Inaddition, the minimum of the resonance also changes due to a change inacoustic impedance mismatch and can be related to liquid density.Electronic circuitry has been developed that can monitor all threeparameters in a continuous manner. The shift in the baseline for the twoplots (water and crude oil) is due to the fact that the acousticimpedance is different for the two fluids. The minimum value of theresonance provides a measure of the fluid density that can be derivedfrom the acoustic impedance mismatch between the pipe wall and the fluidinside.

Thus, it is seen that frequency location of the resonance peaks variesas a function of both the composition of the fluid and its flow rate. Ifa flow meter is desired, the composition must be determined to beconstant; this can be achieved by monitoring the peak spacing todetermine that the sound speed of the fluid remains relatively constantfor in-situ calibration. The calibration can also be performed using asmall section of the same pipe and a known liquid elsewhere in anyflowing system to derive the calibration information. In the flowcalibration, any resonance peak in a desired frequency range (preferablyin the frequency range in the middle of two wall resonance peaks) ismonitored as a function of the liquid flow. The wall resonance peakpositions are determined by the wall thickness. The present apparatuscan be calibrated for both high and low sensitivity measurements asfollows: For low frequencies (approximately 1 MHz), the shift of theresonance peaks is smaller than the shift observed at much higherfrequency (approximately 10 MHz). By observing multiple frequencyranges, it is possible to obtain different levels of sensitivity. Thiscalibration process is no different than for other transit-timeultrasonic flow meters where the fluid sound speed is to be determined.Once the apparatus is calibrated for flow, then both sound speed (andsound absorption) and fluid flow can be simultaneously monitored ifgreat accuracy in the measurement is not desired. For many practicalapplications, such as flow and composition monitoring in the oil(petroleum products) industry, an oil flow calibration provides adequateaccuracy. It is also possible, in principle, to extend the flowcalibration from one liquid, for example, water to oil. FIG. 8illustrates the difference in the resonance peaks for oil and water. Thewidths of the resonance peaks are different for the two liquids, andeach liquid can be identified based on the resonance characteristics ofjust a single resonance peak. Therefore, once the calibration for flowis completed for oil and separately for water, it is possible toextrapolate the flow rate when the flowing fluid is a combination of thetwo liquids because this quantity is intermediate between the twocalibrations. This is possible because the composition can be monitoredfrom a measurement of the peak spacing or by FFT of the resonance data,whereas the flow is measured by tracking the position of a singleresonance peak. These two measurements are independent of each other toa large extent in practice.

The present invention provides information at both low and high flowrates. Since the frequency shift of the peaks due to flow increases withfrequency, for low flow rates it is convenient to use a higher frequencyrange (≧5 MHz) where a small flow rate produces a measurable shift inpeak frequency or phase shift of any selected resonance peak. Bycontrast, for higher flow rates, the resonance peak shift can be largeand one may lose track of the selected peak which is equivalent toexceeding a 360-degree phase shift. In this case, it is appropriate toobserve the data at a lower frequency region (≈1 MHz). The appropriatefrequency ranges depend on the particular pipe geometry and may bedetermined during the initial calibration process where a wide-bandfrequency scan is employed to determine the characteristics of the pipe(see FIG. 3 hereof). As mentioned hereinabove, it is preferable to usethe frequency regions between two wall resonance frequencies for bothflow and composition monitoring.

For a calibration of the system for flow, measurements (receiver signalamplitude and phase difference) are made with a flowing liquid forseveral flow values and the entire frequency spectrum is monitored. Oncethis is done, the calibration information for the low and high frequencyranges are extracted from these spectra and stored in themicrocontroller as terms of simple equations. From this any value canthen be interpolated for actual measurement.

Once the apparatus is calibrated for flow, both sound speed (and soundabsorption) and fluid flow can be simultaneously monitored if greataccuracy in the measurement is not desired. For flow and compositionmonitoring of petroleum products, a simple flow calibration with oil canprovide adequate monitoring. It is also possible, in principle, toextend the flow calibration from one liquid, for example, water to oil.FIG. 8 shows the difference in the resonance peaks for oil and water,and the liquid can be readily identified from the resonancecharacteristics of a single peak. Once the calibration for flow isperformed with oil and then with water, it is possible to correct theflow when the flowing fluid is a combination of any two because themeasured results will be between those for either liquid. This ispossible because the composition is monitored by measuring the peakspacing or FFT of the resonance data, whereas the flow is measured bytracking the position of a single resonance peak.

These two measurements are independent of each other. The foregoingdescription of the invention has been presented for purposes ofillustration and description and is not intended to be exhaustive or tolimit the invention to the precise form disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

REFERENCES

-   1. U.S. Pat. No. 5,767,407 for “Noninvasive Identification Of Fluids    By Swept-Frequency Acoustic Interferometry,” which issued to    Dipen N. Sinha on Jun. 16, 1998.-   2. F. Eggers and Th. Funck, “Ultrasonic relaxation spectroscopy in    liquids”, Naturwissenschaften 63, 280 (1976).-   3. Dipen N. Sinha and Greg Kaduchak, “Noninvasive Determination of    Sound Speed and Attenuation in Liquids,” Experimental Methods in the    Physical Sciences, Volume 39, Academic Press (September 2001).-   4. U.S. Pat. No. 5,606,130 for “Method For Determining The Octane    Rating Of Gasoline Samples By Observing Corresponding Acoustic    Resonances Therein” which issued to Dipen N. Sinha and Brian W.    Anthony on Feb. 25, 1997-   5. U.S. Pat. No. 5,886,262 for “Apparatus And Method For Comparing    Corresponding Acoustic Resonances in Liquids” which issued to    Dipen N. Sinha on Mar. 23, 1999.-   6. Zhijing Wang and Amos Nur, J. Acoust. Soc. Am. 89, 2725 (1991).-   7. Z. Wang and A. Nur, Geophysics 55, 723 (1990).

1-28. (canceled)
 29. A method for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel which comprises thesteps of: (a) applying a continuous periodic acoustical signal to theoutside of the vessel such that the acoustical signal is transferred tothe flowing fluid, thereby generating vibrational resonance features;(b) detecting the vibrational features generated in the flowing liquid;(c) sweeping the continuous periodic acoustical signal through a chosenfrequency range which includes a portion of one vibrational resonancefeature; (d) measuring the phase of the vibrational resonance featurerelative to that for the continuous periodic acoustical signalgenerating thereby a phase difference; (e) determining the compositionof the fluid; and (f) correcting the phase difference for thecomposition of the fluid, whereby the flow rate of the fluid isdetermined.
 30. An apparatus for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said pipe for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein, andfor detecting the generated vibrational pattern; (b) a sweep generatorfor sweeping said first transducer through a chosen frequency rangewhich includes a portion of one vibrational resonance feature; (c) meansfor measuring the phase of the vibrational resonance feature relative tothat for the continuous periodic acoustical signal generating thereby aphase difference; (d) means for determining the composition of thefluid; and (e) a data processor for recording the phase difference andcorrecting the phase difference for the composition of the fluid,whereby the flow rate of the fluid is determined
 31. A method formonitoring the composition of a fluid flowing through a vessel at a flowrate which comprises the steps of: (a) applying a continuous periodicacoustical signal to the outside of the vessel such that the acousticalsignal is transferred to the flowing fluid, thereby generatingvibrational resonance features; (b) detecting the vibrational featuresgenerated in the flowing liquid; (c) sweeping the continuous periodicacoustical signal through a chosen frequency range which includes aportion of one vibrational resonance features; (d) measuring the phaseof the vibrational resonance feature relative to that for the continuousperiodic acoustical signal generating thereby a phase difference; (e)determining the flow rate of the fluid; and (f) correcting the phasedifference for the flow rate of the fluid, whereby changes in thecomposition of the fluid are identified.
 32. An apparatus for monitoringthe concentration of a fluid flowing through a vessel at a flow ratewhich comprises in combination: (a) a first transducer in acousticcontact with the outside surface of said pipe for applying a continuousperiodic acoustical signal to the outside of said vessel such that theacoustical signal is transferred to said flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein, and for detecting the generated vibrational pattern;(b) a sweep generator for sweeping said first transducer through achosen frequency range which includes a portion of one vibrationalresonance feature; (c) means for measuring the phase of the vibrationalresonance feature relative to that for the continuous periodicacoustical signal generating thereby a phase difference; (d) a flowmeter for determining the flow rate of the fluid; and (e) a dataprocessor for recording the phase difference and correcting the phasedifference for the composition of the fluid.